1. Field of Invention
The present disclosure relates to downhole pumping systems submersible in well bore fluids. More specifically, the present disclosure concerns monitoring viscosity of fluid flowing to a downhole pump and adjusting pump operating parameters depending on the viscosity measurement.
2. Description of Prior Art
Submersible pumping systems, as a means of artificial lift method, are often used in hydrocarbon producing wells for pumping fluids from within the well bore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used in this application employs an electrical submersible pump (ESP), another example of artificial lift method is a positive cavitation pump (PCP) system. ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via cable or wireline. Typically, the pumping unit is disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect.
With reference now to FIG. 1, an example of a submersible ESP disposed in a well bore is provided in a partial cross sectional view. In this embodiment, a downhole pumping system 7 is shown within a cased wellbore 5. The pumping system 7 is suspended within the wellbore 5 on production tubing 11. The downhole pumping system 7 comprises a pump section 12, a seal section 16, and a motor 18. The seal section 16 forms an upper portion of the motor 18 and is used for equalizing lubricant pressure in the motor 18 with the wellbore hydrostatic pressure. Energizing the motor 18 then drives a shaft (not shown) coupled between the motor 18 and the pump section 12. Impellers are coaxially disposed on the shaft and rotate with the shaft within respective diffusers formed into the pump 12.
Centrifugal action of the impellers produces a localized reduction in pressure in the diffuser thereby inducing fluid flow into the diffuser. In this embodiment, a series of inlets 14 are provided on the pump housing 10 wherein formation fluid can be drawn into the inlets and into the pump section 12. The series of inlets 14 can be an integral part of the pump or a separate intake/gas separator. The source of the formation fluid, which is shown by the arrows, are perforations 22 formed through the casing 6 of the wellbore 5 and into a surrounding hydrocarbon producing formation 20. Thus the fluid flows from the formation 20, past the motor 18 on its way to the inlets 14. The flowing fluid contacts the housing of the motor 18 and draws heat from the motor 18, providing cooling to the motor 18.
In spite of the heat transfer between the fluid and the motor 18, over a period of time the motor 18 may become overheated. This is especially a problem when the fluid has a high viscosity, a low specific heat, and a low thermal conductivity. This is typical of highly viscous crude oils. The motor 18 may be forced to operate at an elevated temperature, past its normal operating temperature, in order to reject the internally generated heat. This temperature upset condition can reduce motor life and results in a reduction in operational times of the pumping system.
Pump horsepower and torque requirements increase with the increase in the viscosity of the fluid. When ESP system pumps highly viscous fluid, the increase in horsepower requirements leads to higher torque and overloading of the motor 18. This may result in breaking of shaft of the pump and/or the motor, as well as additional heat generation in the motor.